Streaming media is multimedia that is constantly received by, and normally presented to, an end-user (using a client) while it is being delivered by a streaming provider (using a server). Several protocols exist for streaming media, including the Real-time Streaming Protocol (RTSP), Real-time Transport Protocol (RTP), and the Real-time Transport Control Protocol (RTCP), which are often used together. The Real Time Streaming Protocol (RTSP), developed by the Internet Engineering Task Force (IETF) and created in 1998 as Request For Comments (RFC) 2326, is a protocol for use in streaming media systems, which allows a client to remotely control a streaming media server, issuing VCR-like commands such as “play” and “pause”, and allowing time-based access to files on a server.
The sending of streaming data itself is not part of the RTSP protocol. Most RTSP servers use the standards-based RTP as the transport protocol for the actual audio/video data, acting somewhat as a metadata channel. RTP defines a standardized packet format for delivering audio and video over the Internet. RTP was developed by the Audio-Video Transport Working Group of the IETF and first published in 1996 as RFC 1889, and superseded by RFC 3550 in 2003. The protocol is similar in syntax and operation to Hypertext Transport Protocol (HTTP), but RTSP adds new requests. While HTTP is stateless, RTSP is a stateful protocol. A session ID is used to keep track of sessions when needed. RTSP messages are sent from client to server, although some exceptions exist where the server will send messages to the client.
RTP is usually used in conjunction with RTCP. While RTP carries the media streams (e.g., audio and video) or out-of-band signaling (dual-tone multi-frequency (DTMF)), RTCP is used to monitor transmission statistics and quality of service (QoS) information. RTP allows only one type of message, one that carries data from the source to the destination. In many cases, there is a use for other messages in a session. These messages control the flow and quality of data and allow the recipient to send feedback to the source or sources. RTCP is a protocol designed for this purpose. RTCP has five types of messages: sender report, receiver report, source description message, bye message, and application-specific message. RTCP provides out-of-band control information for an RTP flow. RTCP partners with RTP in the delivery and packaging of multimedia data, but does not transport any data itself. It is used periodically to transmit control packets to participants in a streaming multimedia session. One function of RTCP is to provide feedback on the quality of service being provided by RTP. RTCP gathers statistics on a media connection and information such as bytes sent, packets sent, lost packets, jitter, feedback, and round trip delay. An application may use this information to increase the quality of service, perhaps by limiting flow or using a different codec or bit rate.
One problem with existing media streaming architectures is the tight coupling between server and client. The stateful connection between client and server creates additional server overhead, because the server tracks the current state of each client. This also limits the scalability of the server. In addition, the client cannot quickly react to changing conditions, such as increased packet loss, reduced bandwidth, user requests for different content or to modify the existing content (e.g., speed up or rewind), and so forth, without first communicating with the server and waiting for the server to adapt and respond. Often, when a client reports a lower available bandwidth (e.g., through RTCP), the server does not adapt quickly enough causing breaks in the media to be noticed by the user on the client as packets that exceed the available bandwidth are not received and new lower bit rate packets are not sent from the server in time. To avoid these problems, clients often buffer data, but buffering introduces latency, which for live events may be unacceptable.
In addition, the Internet contains many types of downloadable media content items, including audio, video, documents, and so forth. These content items are often very large, such as video in the hundreds of megabytes. Users often retrieve documents over the Internet using HTTP through a web browser. The Internet has built up a large infrastructure of routers and proxies that are effective at caching data for HTTP. Servers can provide cached data to clients with less delay and by using fewer resources than re-requesting the content from the original source. For example, a user in New York may download a content item served from a host in Japan, and receive the content item through a router in California. If a user in New Jersey requests the same file, the router in California may be able to provide the content item without again requesting the data from the host in Japan. This reduces the network traffic over possibly strained routes, and allows the user in New Jersey to receive the content item with less latency.
Unfortunately, live media often cannot be cached using existing protocols, and each client requests the media from the same server or set of servers. In addition, when streaming media can be cached, it is often done by specialized cache hardware, not existing and readily available HTTP-based Internet caching infrastructure. The lack of caching limits the number of parallel viewers and requests that the servers can handle, and limits the attendance of a live event. The world is increasingly using the Internet to consume up to the minute live information, such as the record number of users that watched live events such as the opening of the 2008 Olympics via the Internet. The limitations of current technology are slowing adoption of the Internet as a medium for consuming this type of media content.